LABOUR HQ has insisted it is not interested in playing "Westminster games" over a possible alliance with the SNP as Paddy Ashdown warned that the Nationalists did not want to go south to help Westminster but to destroy it.

As speculation about the possibility of a post-election Lab-SNP arrangement continues to run through the election campaign - a Labour peer suggested a deal with the Nationalists would be "pragmatic politics" - the party leadership responded tetchily to any question that involved their Scottish opponents.

Asked if Ed Miliband would respond to the announcement at the weekend that Nicola Sturgeon wanted to raise the minimum wage by £2 to £8.70 an hour by 2020 - Labour's policy is to raise it to £8 an hour - a source close to the Leader of the Opposition said: "We have set out our own policy with the minimum wage and we are not interested in any negotiations or Westminster games."

Speaking at Labour's business manifesto launch in the City of London, where outside Tory activists wore Alex Salmond face masks and carried posters, declaring: "I'll prop you up Ed", the source claimed the Conservatives were "desperate to focus on what might or might not happen" after the election.

"We are not interested in talking about other parties' programmes or some sort of Conservative Party invention where we are involved in some kind of negotiation. We are not, full stop," he added.

The prospect of a Labour-SNP alliance led to warnings being aired not only by Lord Ashdown but also by London Mayor Boris Johnson.

The former Liberal Democrat leader insisted Ms Sturgeon's party had just one aim. "They're not coming south to help Westminster work, they're coming south as a Scottish raiding party to burn Westminster down and to make the thing dysfunctional," he declared.

The former UN High Representative for Bosnia and Herzegovina went on: "I watched it happen in the Balkans, by the way. You first of all make it dysfunctional and then you have a case for breaking away; that's what their aim is."

The Lib Dem peer made clear he was not comparing Mr Salmond to former Serbian President Slobodan Milosevic, who died in 2006, facing war crimes charges.

But his comments sparked controversy with Twitter users branding them "outrageous".

Meantime, Mr Johnson described the prospect of a Labour-SNP arrangement as "chilling the blood of sensible people across the UK".

"Salmond at Westminster," argued the Conservative Mayor, "would run rings round Miliband and in any kind of Labour-SNP coalition it is all too easy to see how the Scottish tail would wag the English dog."

He claimed the likes of Ms Sturgeon and her SNP colleagues would spend their time in any governing pact engaged in an effort to "tease, bait, goad and generally wind up the English until the patience snaps".

In a separate development, Labour peer Baroness Prosser when pressed over whether or not her party might be prepared to do a deal with the SNP on a case by case basis, said: "That is just how pragmatic politics works."

Stewart Hosie, the Nationalists' deputy leader, seized on her words and said: "Margaret Prosser's comments suggest Labour are planning for an election result in which the SNP will hold the balance of power in the next parliament."